Power feed circuit providing both on-hook and off-hook power for telephone subscriber loop

ABSTRACT

A power feed circuit provides power for a telephone subscriber loop connected to a unit of terminal equipment. The power feed circuit includes a first current supply circuit for supplying power to the subscriber loop when the unit of terminal equipment is in an on-hook condition and when the unit of terminal equipment is in an off-hook condition, and a second current supply circuit for supplying power to the subscriber loop only when the unit of terminal equipment is in the off-hook condition. Each current supply circuit includes a respective battery, transistor and bias network. The first current supply circuit is connected to the subscriber loop via a diode which is reverse-biased when the unit of terminal equipment is in the on-hook condition. The power feed circuit may be usefully applied in an Optical Network Unit (ONU) of a &#34;fiber-in-the-loop&#34; telephone system or in a Subscriber-Cable Interface Unit (SCIU) of a cable telephony system.

BACKGROUND OF THE INVENTION

This invention is concerned with circuitry for providing a local loopconnection between a telephone central office and a subscriber'spremises, and is more particularly concerned with power feed circuitryfor the subscriber loop.

It has been proposed to implement a portion of the signal path betweenthe central office and subscriber premises in the form of opticalfibers. In such systems, known as "fiber-in-the-loop" (FITL) systems, anOptical Network Unit (ONU) is connected between the subscriber side ofthe optical fiber and the metal wire pair which completes the loop tothe subscriber premises.

It has also been proposed to combine distribution of telephone signalswith video signals in a broad-band coaxial cable network. An example ofsuch a so-called cable telephony system is described in U.S. Pat. No.5,351,234, which has a common inventor with the present application.(The disclosure of the '234 patent is incorporated herein by reference.)In a cable telephony system, a Subscriber-Cable Interface Unit (SCIU) isinstalled between the coaxial cable and the metal wire pair whichcompletes the subscriber loop.

It is customary to include in the ONU of an FITL system, or in the SCIUof a cable telephony system, a power feed circuit which provides atleast some of the power required for the terminal equipment installed atthe customer premises. FIG. 1 schematically illustrates such a powerfeed circuit.

As seen from FIG. 1, the power feed circuit includes a power supply Swhich is connected to a telephone 20 or other terminal equipmentinstalled at the customer premises through a loop interface unit 22 anda tip and ring wire pair 24 which constitutes the loop connectionbetween the interface unit 22 and the telephone 20.

The loop interface unit provides voice band transmission, ringing, andother associated functions, including determining whether the telephone20 is in an on-hook or off-hook condition by monitoring the loopcurrent, I_(L). The power supply S typically contains a battery orconstant-voltage source, which is connected in parallel to serve therespective loops of a number of different subscribers (although only onesuch loop is shown in FIG. 1). The power supply S also includes acurrent-limiting resistor for each loop.

According to applicable standards promulgated by Bellcore or ANSI, theloop voltage V_(L) to be provided by the power feed circuit should be noless than 9 V when the customer premise equipment is in an off-hookcondition, and no less than 21 V when the customer premise equipment isin an on-hook condition. The current to be provided for the off-hookcondition must be no less than 20 mA, assuming that the loop andcustomer premise equipment meet the requirement that the off-hookresistance is no greater than 450Ω. For the on-hook condition theresistance must be at least 5MΩ, so that the requirement for on-hookloop current is only 4.2 μA.

The cost of providing loop power is an important element in the overallcost of the FITL or cable telephony system, and it would therefore bedesirable to achieve greater efficiency than that found in conventionalbattery plus resistor power feed circuits. It is also believed that apower feed circuit capable of supplying substantially more than theminimum power required for the on-hook condition may provide significantadvantages. For example, a "loop reconnection device" disclosed in U.S.Pat. No. 5,764,754 (application Ser. No. 08/362,613, filed Dec. 22,1994, and having a common inventor with this application) might requiresubstantially more power than that provided for in the existing advisorystandard for on-hook power. The disclosure of the '613 patentapplication is incorporated herein by reference.

It is therefore a primary object of the invention to provide animprovement in subscriber loop power feed circuits of the type employedin fiber-in-the-loop and cable telephony systems.

It is an additional object of the invention to reduce the powerdissipation and cost of a telephone subscriber loop system in which aspecified amount of DC power is required to be supplied to thesubscriber equipment when it is in an on-hook condition.

It is a further object of the invention to provide a power feed circuitwhich is capable of substantially exceeding the recommended criteria forproviding on-hook power without adversely affecting off-hook powerdissipation.

SUMMARY OF THE INVENTION

In accordance with the principles of the present invention, the aboveand other objects are realized by provision of a power feed circuit fora telephone subscriber loop which is connected to a unit of terminalequipment, the unit of terminal equipment being switchable between anon-hook condition and an off-hook condition, the power feed circuitincluding a first current supply circuit for supplying power to thesubscriber loop when the unit of terminal equipment is in the on-hookcondition and when the unit of terminal equipment is in the off-hookcondition, and a second current supply circuit which supplies power tothe subscriber loop only when the unit of terminal equipment is in theoff-hook condition. Preferably the first and second current supplycircuits are connected in parallel to the subscriber loop and eachincludes a respective battery and a respective resistor-transistornetwork. Also, it is preferred that the second current supply circuit beconnected to the subscriber loop via a diode which is reverse-biasedwhen the unit of terminal equipment is in its on-hook condition.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features, advantages and aspects of the presentinvention will become more apparent upon reading the following detaileddescription in conjunction with the accompanying drawings, in which:

FIG. 1 schematically illustrates a conventional subscriber loop powerfeed circuit;

FIG. 2 is a schematic block illustration of a subscriber loop power feedcircuit provided in accordance with the invention;

FIG. 3 shows ideal current source characteristics for the power feedcircuit of FIG. 2;

FIG. 4 graphically illustrates the amount of power supplied by thecircuit of FIG. 2, as a function of the loop resistance;

FIG. 5 is a detailed schematic representation of a preferred embodimentof the power feed circuit of FIG. 2;

FIG. 6 illustrates the current supply characteristic of an on-hooksupply portion of the circuit of FIG. 5;

FIG. 7 illustrates circuit component values set according to anapproximate design process for the on-hook supply portion of the circuitof FIG. 5;

FIG. 8 graphically represents the current source characteristic of theoff-hook supply portion of the circuit of FIG. 5;

FIG. 9 graphically represents the combined current supply characteristicof the circuit of FIG. 5;

FIG. 10 shows circuit component values obtained as a result of anapproximate design process for the off-hook supply portion of thecircuit of FIG. 5;

FIG. 11 shows circuit component values obtained as a result of a moreprecise design procedure for the circuit of FIG. 5;

FIG. 12 is a schematic representation of an alternative preferredembodiment of the circuit of FIG. 2, with a lower on-hook power supplycapability than the circuit of FIG. 5;

FIG. 13 shows an ideal current source characteristic of the circuit ofFIG. 12;

FIG. 14 schematically illustrates in cross-section a semiconductorimplementation of the transistors and the diode included in the powerfeed circuit of FIG. 5;

FIG. 15 is a schematic block illustration of a subscriber loop powerfeed circuit according to an alternative embodiment of the invention;and

FIG. 16 shows an ideal current source characteristic for the circuit ofFIG. 15.

DETAILED DESCRIPTION

FIG. 2 shows a power feed circuit in accordance with the presentinvention, in which the conventional power supply S of FIG. 1 isreplaced with an improved power supply S'. The power supply S' includesan off-hook supply S₁ and an on-hook supply S₂ which are connected inparallel to the subscriber loop 24 via the conventional loop interfaceunit 22. The off-hook supply S₁ is connected to the subscriber loopthrough a diode D which is reverse-biased when the telephone 20 is inthe on-hook condition, so that essentially all power for the loop isderived from the on-hook supply S₂ at that time. When the telephone 20is off-hook, substantial power is also derived from the off-hook supplyS₁, in addition to power supplied from the on-hook supply S₂. In otherwords, the on-hook supply always provides power for the subscriber loop,whereas the off-hook supply provides substantial power only when thetelephone 20 is in its off-hook condition.

If the power supplies S₁ and S₂ were ideal circuits, each would have thecurrent-voltage characteristic of a constant-current source, as shown,respectively, in FIGS. 3A and 3B. Consequently, the supplies S₁ and S₂would be able, respectively, to deliver maximum power levels V₁ ·I₁ andV₂ ·I₂, to loads V₁ /I₁ and V₂ /I₂. These characteristics are ideal inthe sense that the supplies S₁ and S₂ need not be designed to handlemore power than the above maximum; if the load were a short circuit, theamounts of power dissipated by supplies S₁ and S₂ would still be,respectively, V₁ ·I₁ and V₂ ·I₂.

If an ideal diode were part of the circuit along with ideal supplies,the resulting current-voltage characteristic at the subscriber loopwould be as shown in FIG. 3(c). For values of the loop voltage V_(L)between zero and V₁, the diode D is forward-biased, with zero voltagedrop, and the resulting current is the sum of the characteristic curvesfor supplies S₁ and S₂. For values of V_(L) greater in magnitude than(i.e., more negative than) V₁, the diode D is reverse-biased, with zerocontribution to the resulting current. In this voltage range, theresulting loop current I_(L) is simply the current provided by supplyS₂.

FIG. 3(c) also shows typical off-hook and on-hook operating points A andB which are points where the characteristic of the combined supplyintersects, respectively, with load lines having slopes corresponding tothe off-hook and on-hook values of the loop resistance V_(L) /I_(L).

According to the standards referred to above, the voltage V₁ providedfor the off-hook condition should be at least 9.0 V, and the voltage V₂for the on-hook condition should be at least 21 V. Further, the sum I₁+I₂ should be not less than 20 mA. The on-hook current level I₂ may bechosen to meet a desired on-hook power level in excess of that requiredfor the standard current requirement of 4.2 μA. With the circuit designdescribed herein, I₂ can be much larger than 4.2 μA.

For the ideal model of FIG. 3(c), FIG. 4 shows how the power deliveredto the loop, P_(L) =V_(L) I_(L), depends in general upon the loopresistance, R_(L) =V_(L) /I_(L). For small values of R_(L) constitutingthe off-hook region, the circuit delivers a constant current and hence apower that is proportional to R_(L), up to a peak at R_(L) =V₁ /(I₁+I₂). In this region, the power delivered to the loop is derived fromboth supplies S₁ and S₂, in the proportions I₁ (I₁ +I₂) and I₂ /(I₁ +I₂)respectively. For larger values of R_(L), there is a region betweenR_(L) =V₁ /(I₁ +I₂) and R_(L) =V₁ /I₂ in which the power delivered tothe loop is still derived from both supplies S₁ and S₂, but the powerfrom supply S₂ remains constant at V₁ I₂ while that from supply S₁ isgiven by (V₁ ² /R_(L))-V₁ I₂. This region ends when diode D becomesback-biased. Beyond that point, there is no power coming from supply S₁.Here, the power from supply S₂ is given by I₂ ² R_(L) up to another peakat R_(L) =V₂ /I₂. For still higher values of R_(L), the loop power(still all derived from supply S₂) is V₂ ² /R_(L). This is the region inwhich the loop voltage meets the off-hook minimum requirement.

The peaks are at end points of ranges of values of R_(L) for which therequirements for off-hook current and for on-hook voltage are met. Forvalues of R_(L) between these limits, the loop will be off-hook asjudged by the current detector (not shown) in the loop interface unit22, but the loop voltage will be intermediate between V₁ and V₂. Thepower feed circuit should be designed so that this intermediate range ofvalues of R_(L) does not include any values that are expected in actualapplications. This can readily be achieved for an ONU or SCIU, since anONU or SCIU is only required to serve a relatively short loop.

The on-hook peak power may be greater or less than the off-hook peak.The ratio is (V₂ /V₁)/(1+I₁ /I₂), which reaches its greatest value whenI₂ =I₁ +I₂, or I₁ =0. In this limiting case only S₂ is operative.

One possible design would provide for equal amounts of on-hook andoff-hook power. This requires

    I.sub.2 =(V.sub.1 /V.sub.2) (I.sub.1 +I.sub.2).

For the typical values given above, the peak amounts of loop power willbe equal, at 180 mW, for I₂ =8 4/7 mA. The "forbidden" range of valuesof R_(L) between the peaks is then from 450Ω to 2,450Ω.

The amount of on-hook power that is required may well be less than theoff-hook power. As may be seen from FIG. 4, when I₁ +I₂, V₁, and V₂ arefixed, the value of R_(L) at the on-hook peak 28 is inverselyproportional to I₂ and hence to the magnitude of the on-hook peak power.For example, for the peak to occur at the ANSI-specified value of 5MΩrequires I₂ =4.2 μA, for an on-hook peak of 88.2 μW. An actual circuitdesign presented below provides substantially more on-hook power, 42 mW,supplying I₂ =2 mA at R_(L) =10,500Ω.

FIG. 5 shows an embodiment of the circuit of FIG. 2 that approximatesthe ideal characteristic of FIG. 3(c). It employs two batteries, E₁ andE₂, two resistor-transistor networks, N₁ and N₂, and a semiconductordiode D. The elements E₁ and N₁ constitute the off-hook supply S₁ ofFIG. 2, while E₂ and N₂ constitute the on-hook supply S₂. Batteries E₁and E₂ may serve several loops, with a respective pair of networks N₁and N₂ for each loop.

Since the actual circuit incorporates dissipative elements, it requiresgreater operating power than the ideal model in order to meet thespecified on- and off-hook voltage and current conditions. The greatestoperating power is drawn at the short-circuit condition, when all poweris dissipated in networks N₁, N₂, and diode D while none is delivered tothe loop.

Network N₁ is formed of an npn transistor Q₁ and resistors R_(E1), R₁₁and R₂₁. The diode D is connected between a loop terminal 30 of thepower feed circuit and a collector terminal 32 of the transistor Q₁. Thediode D has a polarity for conducting current only in the direction fromthe loop terminal 30 to the collector terminal 32 of transistor Q₁.

The resistor R_(E1) is connected between an emitter terminal 34 oftransistor Q₁ and a negative terminal 36 of battery E₁. Resistor R₁₁ isconnected between a base terminal 38 of transistor Q₁ and the negativeterminal 36 of battery E₁. Resistor R₂₁ is connected between the baseterminal 38 of transistor Q₁ and another loop terminal 40 of the powerfeed circuit. The positive terminal 42 of the battery E₁ is alsoconnected to loop terminal 40, and resistors R₁₁ and R₂₁ form a voltagedivider for biasing the base of the transistor Q₁.

The network N₂ of the on-hook supply S₂ is formed of npn transistor Q₂and resistors R_(E2), R₁₂ and R₂₂. The collector terminal 44 oftransistor Q₂ is connected to the loop terminal 30. The resistor R_(E2)is connected between the emitter terminal 46 of transistor Q₂ and thenegative terminal 48 of battery E₂. Resistor R₁₂ is connected betweenthe negative terminal 48 of battery E₂ and the base terminal 50 oftransistor Q₂. Resistor R₂₂ is connected between the base terminal 50 oftransistor Q₂ and the loop terminal 40, to which the positive terminal52 of the battery E₂ is also connected. The resistors R₁₂ and R₂₂consequently form a voltage divider for biasing the base of transistorQ₂.

When the subscriber loop is in the off-hook condition, the loop currentI_(L) is a combination of the respective collector currents I_(C1) andI_(C2) of the transistors Q₁ and Q₂. When the loop is in the on-hookcondition, the diode D is reverse-biased, and I_(L) is entirely providedby the collector current of transistor Q₂.

The transistors Q₁ and Q₂ may each be realized by a type 2N4401general-purpose silicon transistor available from Motorola. The diode Dmay also be formed by a transistor of the same type by connecting itsbase and collector terminals together to place its junctions inparallel. Those of ordinary skill in the art will recognize that othertypes of transistors and diodes may also be used. Also, as will bediscussed below, the devices in the circuit of FIG. 5 may be fabricatedas an integrated circuit, possibly also integrated with elements of theloop interface unit 22.

For the purposes of a design example which will now be presented, itwill be assumed that the circuit is to operate at 20° C. and the deviceswill be assumed to exhibit nominal values, which can be found atAppendix K, pages 1073-1077 of the Art of Electronics, by Horowitz andHill, Second Edition, 1989. Compliance with the nominal desiredcharacteristics will also be assumed. Those of ordinary skill in the artwill be able to modify the calculations set forth below to take intoaccount tolerances in the device parameters, as well as other operatingconditions.

DESIGN OF ON-HOOK SUPPLY

In a first part of this design example, the on-hook supply S₂ will beconsidered.

Since even a real semi-conductor diode is essentially ideal when it isreverse-biased (i.e., there is essentially no reverse current), thetotal loop current, as noted above, is the same as I_(C2) (the collectorcurrent of transistor Q₂) when the loop is in an on-hook condition. Thevalues for the battery E₂ and the resistors R_(E2), R₁₂ and R₂₂ cantherefore be selected to provide V₂ and I₂ to meet the desired minimumlimits of 21 V and 2 mA. Initially, approximate design parameters willbe proposed and then a more specific design corresponding to a moreexact circuit model will be developed.

FIG. 6 shows a current supply characteristic which substantiallycorresponds to the characteristic of FIG. 3(b) but can actually beachieved with real circuit elements. The actual loop voltage V_(L) andcurrent I_(L) are given by the point B' at which the characteristic ofFIG. 6 intersects with the on-hook load line. The initial part of thedesign task is to provide an actual characteristic that satisfies thelimiting on-hook current and voltage values I₂ =2 mA and V₂ =21 V. Theopen-circuit value of V_(L) is the potential provided by battery E₂,which is somewhat greater than V₂ ; the short-circuit value of I_(L) ;I_(C2MAX), is somewhat greater than I₂.

The elements of S₂ should be chosen so that the power delivered to theloop, V_(L) I_(L), is greatest at the point V₂,I₂. The condition forthis is ##EQU1## which requires ##EQU2## While this requirement can beevaluated exactly only when V_(L) (I_(L)) is known, the point where itis satisfied will clearly be near the "knee" of the characteristic.

As noted before, resistors R₁₂ and R₂₂ form a voltage divider, whichforward biases the emitter-base junction of Q₂ and reverse biases thecollector-emitter junction. As will be seen, the selection of the valuesfor the resistors R₁₂ and R₂₂ making up the voltage divider is subjectto a trade-off. On one hand, it is desirable to minimize the combinedvalue of these resistors in order to minimize the effect of the basecurrent upon the voltage at the base of transistor Q₂. On the otherhand, it is desirable to maximize the combined value of these resistorsin order to minimize power loss due to current through the voltagedivider.

FIG. 7 shows circuit element values chosen in an attempt to approximatean optimal design. For on-hook values of the loop resistance R_(L), Q₂should be in or near saturation, so that the collector-emitter voltageV_(CE2) is minimized. Consequently, the loop voltage V_(L) differs fromthe voltage provided by battery E₂ mainly by the quantity I_(E2)·R_(E2). The value of E₂ should therefore be chosen to be somewhathigher than the required voltage V₂, and R_(E2) should be quite smallcompared with the quotient obtained by dividing the value of E₂ by thedesired on-hook loop current I₂. For example, to achieve V₂ =21 V, itwould be reasonable to design for a (24±1) V battery so that theworst-case value of E₂ is 23 V. Then for I₂ =2 mA, a reasonable choicemight be R_(E2) =500Ω, for which I_(E2) R_(E2) =1 V. Then V₂ will be atleast 21 V as long as the collector-emitter voltage of Q₂ in saturationis less than 1 V.

In order to reduce the power consumed by the bias network, the value of(R₁₂ +R₂₂) should be appreciably larger than E₂ /I₂ : that is,appreciably larger than 11,500Ω. However, (R₁₂ +R₂₂) should be smallenough that, when Q₂ is out of saturation, its emitter-base voltage ispredominantly determined by the voltage divider action of R₁₂ and R₂₂and is relatively unaffected by the base current. For h_(FE) =100, theseconditions could be satisfied by choosing R₂₂ =50,000Ω which wouldprovide a current through the voltage divider that is roughly twentytimes the nominal base current of 20 μA. Then, since the base-emittervoltage is about 0.7 V, R₁₂ should be about 1.7 V/0.406 mA=4,200Ω.

Maximum power is drawn from E₂ when the loop is short-circuited.Considering base current to be negligible, this would be

    P.sub.2MAX =(23 V) (2.0 mA)+(23 V).sup.2 /54,200Ω=56 mW

The power delivered to the loop is 42 mW, which meets the designobjective.

A more precise set of circuit element values can be derived by using astandard transistor model and typical transistor parameters. One resultof this approach will be a more accurate formula for the value of R₁₂that will produce a given short-circuit current I_(C2MAX). Followingfrom this result is an expression for the greatest operating power. Athird result is a form of the condition for maximum loop power (equation2) that can be solved numerically for any circuit element values, usingstandard data concerning the relationship between Q₂ 'scollector-emitter voltage V_(CE2) and base current I_(B2).

For typical solutions, I₂ is found to be very nearly equal to I_(C2MAX).Moreover, in circuits designed for different values of I_(C2MAX), inwhich resistance values are scaled inversely with I_(C2MAX), the optimumcollector-emitter voltage of Q₂ is found to be less than one volt and isquite independent of I_(C2MAX). Thus, the appropriate value of R₁₂ canbe calculated for any desired I₂.

In general, the circuit equations for on-hook supply S₂ are

    I.sub.12 R.sub.12 =V.sub.BE +(I.sub.C2 +I.sub.B2)R.sub.E2  (3)

and

    E.sub.2 =I.sub.12 R.sub.12 +(I.sub.12 +I.sub.B2)R.sub.22   (4)

where I₁₂ is the current through R₁₂.

Solving equations (3) and (4) to eliminate I₁₂ yields: ##EQU3## whereh_(FE2) is the common-emitter current gain of Q₂ : ##EQU4## According toa frequently used model, the base-emitter voltage V_(BE2) islogarithmically related to the collector current, I_(C2), as follows:##EQU5## Here, V_(T) is the physical constant kT/q, which is 25.3 mV at20° C. An appropriate value of the constant V_(k) may be determined fromthe data sheets for the selected transistor. In the case of theabove-mentioned Motorola type 2N4401 transistor, relevant data curvesare found in FIG. 17, on page 1077 of the Horowitz and Hill referencementioned above. If operation of Q₂ in the active region is to bemodelled with I_(C2) in the vicinity of 10 mA, the curve for V_(BE)(on)shows it to be 0.67 V at I_(C) =10 mA. Hence the model becomes ##EQU6##

For values of collector current that are in the active region, h_(FE2)is considerably greater than unity and is effectively constant. Underthese conditions, equation 5 is satisfied by a unique value of I_(C2),independent of V_(C2). That is, the circuit acts effectively as aconstant-current source, with: ##EQU7## It is these conditions thatobtain at the short-circuit point. It is thus of interest to calculatethe value of R₁₂ that is required to produce a specified short-circuitvalue of I_(C2), I_(C2MAX). For this purpose, equation 5 may berewritten as follows: ##EQU8##

For the same values used above (R_(E2) =500Ω, R₂₂ =50,000Ω, h_(FE) 100,E₂ 23 V), this more accurate formula indicates that R₁₂ should be 4,025Ωfor I_(C2MAX) =2.0 mA, not greatly different from 4,200Ω as calculatedapproximately above.

Equation 8 is not very sensitive to h_(FE). For example, R₁₂ would be4,261Ω or 3,898Ω for h_(FE) equal to 50 or 200 respectively. In therange of h_(FE) specified for Q₂ as 80 or greater, the calculated rangeof R₁₂ is 4,081Ω (h_(FE) =80) to 3,811Ω (h_(FE) =∞). Because thespecified value of I₂ is a minimum, the larger value, 4,081Ω, should bechosen.

As mentioned above, the greatest power is required from on-hook supplyS₂ when the loop is short-circuited. This is P_(2MAX) =E₂ (I_(C2)+I_(B2) +I₁₂), with all variables evaluated for short-circuitconditions. Substituting from equation (4) and rearranging, this becomes##EQU9## In practice, h_(FE) is large and R₁₂ is much less than R₂₂.Thus, there is only a very small difference between this more accurateexpression and the calculation made above in connection with FIG. 7.

Of course, when the load is not a short circuit (and especially forlarger values of V_(C2)), it is no longer true that h_(FE) is constant.However, it turns out that for practical circuit parameters, maximumefficiency occurs when h_(FE) is still at 70-80% of its greatest value.Moreover, at this point, the reduction in h_(FE) is mainly due toincreased base current, so the value of I₂ is only very slightlydifferent from I_(C2MAX). The following analysis makes use of the factthat I_(C2) is essentially constant under the conditions that are ofmost interest: the range of loop resistance between the optimum value V₂/I₂ and a short circuit.

In order to find ##EQU10## we start with:

    V.sub.C2 =E.sub.2 -(.sub.B2 +I.sub.C2)R.sub.E2 -V.sub.CE2 (I.sub.B2) (10)

then: ##EQU11## and: ##EQU12## To find ##EQU13## it is useful to rewriteequation 5 in yet another form: ##EQU14## then: ##EQU15## From equation(6): ##EQU16## Thus, substituting equation 15 in equation 14,substituting the result in equation 12, and rearranging: ##EQU17## Then,for the maximum power condition (equation 2): ##EQU18##

Since in practice V_(T) /I_(C2) <<R_(E2), this may be approximated as:##EQU19##

The left hand side of equation (18) contains parameters of Q₂ that maybe measured or obtained from a manufacturer's data sheets. For a givencircuit design, the optimum operating point (I₂,V₂) may be found byevaluating the right hand side of this equation and then determining thevalues of V_(CE2) and I_(B2) that produce the same numerical value forthe left hand side.

For example, FIG. 16 on p. 1077 of Horowitz and Hill shows how V_(CE)varies as a function of I_(B) in and near the collector saturationregion, with I_(C) as a parameter. Because I_(C) is constrained by thecircuit to be practically constant in the region of interest, the slopeof such a curve may be taken as the total derivative needed in equation18 although of course it is actually a partial derivative. (The validityof this assumption will be verified: once I₂ has been determined, itwill be seen that it is indeed very nearly the same as I_(C2MX).)

Since no curve for the desired value of I_(C) (2.0 mA) is provided, thebehavior of circuits designed for 1.0 mA and 10 mA will be investigated,since these are the closest values for which data is available. In eachcase, R_(E2) and R₂₂ will be scaled in inverse proportion to I_(C), andR₁₂ will be calculated for h_(FE2) =100. Thus, we have:

    ______________________________________                                        I.sub.C2MAX 1.0 mA       10 mA                                                R.sub.E2    1000Ω  100Ω                                           R.sub.22    100,000Ω                                                                             10,000Ω                                        V.sub.EB    .612 V       .670 V (from Eq. 7)                                  R.sub.12    7,959Ω 826.8Ω (from Eq. 8)                            ______________________________________                                    

In each curve, the value of I_(B) appears to approach an asymptoticvalue with increasing V_(CE), as is appropriate when the operatingcondition changes from saturation to the active state. Near theasymptotic value, the curves are represented quite accurately by theexpression ##EQU20## where a and b depend on I_(C). In particular, forthe values of I_(C) of interest, the following parameter values can bederived from the data curves:

    ______________________________________                                        I.sub.C2MAX   1.0 mA        10 mA                                             a             0.0115        0.00816                                           b             0.00142 V     0.00128 V                                         ______________________________________                                    

Expressed in terms of this model, the left hand side of equation 18 is##EQU21## The right hand side depends upon a set of design values chosenfor various circuit elements. For any such choice, after substitutionfrom equation 19, equation 18 can be solved to yield an expression forh_(FE) (I₂), the value of h_(FE) at the optimum operating point, interms of a and b, the parameters of the model. That is, if the righthand side of equation 18 is defined to be -A, ##EQU22##

Equation 20 can be used to determine the relationship between theshort-circuit current, I_(C2MAX), and the current at the optimumoperating point, I₂. First, from equation 5, their ratio can beapproximated as ##EQU23## Equation 21 assumes that V_(BE2) is the samefor the two values of I_(C2) considered, I_(C2MAX) and I₂. Thisassumption is reasonable because, as will be seen, these values ofI_(C2) are nearly the same, and because V_(BE2) is only weakly(logarithmically) related to I_(C2) (equation 6).

Further approximation is justified under the conditions that are underconsideration here. The quantity ##EQU24## occurs in both the numeratorand denominator of equation 21. In the circuit designs proposed above,this quantity is much smaller than either h_(FE) (I₂) or h_(FE)(I_(C2MAX)). This is because

    R.sub.12 <<R.sub.22

and ##EQU25## For example, ##EQU26## and

    h.sub.FE ≅100.

When equation 21 is expanded, assuming the above inequalities andincluding only first order terms, the result is: ##EQU27##

As mentioned above, h_(FE) reaches an asymptotic value, 1/a, as V_(CE)increases into the active region, out of saturation. It is thisasymptotic value that will exist at the short-circuit point, where

    V.sub.CE ≅E.sub.2 -R.sub.E2 I.sub.C2MAX'

That is, the relative error in neglecting the second term in the modelfor 1/h_(FE) is b/(aV_(CE)). Using the empirical values of a and b givenabove for both the 1.0 mA and 10 mA circuits (for both of which V_(CE)=23 V-1 V=22 V), the relative error for the 1.0 mA circuit is 0.6% and,for the 10 mA circuit, 0.7%. These errors are negligible, consideringthat the model itself is only an approximation. Thus, ##EQU28## Makinguse of this as well as equation 20, equation 22 may be evaluated interms of the parameters of the model, with ##EQU29##

Finally, the value of V_(CE) at the optimum operating point may beexpressed in terms of the model through the use of equation 20:

    V.sub.CE (I.sub.2)=√bA                              (24)

The above formulas may be applied in an iterative process, in which adesired value of V₂ is used to calculate a first value of A, which leadsto a value of V_(CE) from equation 24 and hence a new value of V₂. Theprocess is highly convergent so that even a second pass is unnecessary.A first pass produces the following numerical results:

    ______________________________________                                        I.sub.C2MAX 1.0mA     10mA                                                    A           168.4V    173.7V                                                   ##STR1##   0.00290   0.00271                                                  ##STR2##   1.025     1.023     (from Eqs. 22, 23)                             ##STR3##   .49V      .47V                                                    h.sub.FE (I.sub.C2MAX) (= 1/a)                                                            87.0      122.5                                                   h.sub.FE (I.sub.2)                                                                        69.4      92.0      (from Eq. 20)                                 V.sub.2     21.50V    21.52V                                                  V.sub.2 /I.sub.2                                                                          21,500Ω                                                                           2,152Ω                                            ______________________________________                                    

These figures are not substantially changed by iteration. For the 10 mAcase, the results of the first, second, and third passes for V_(CE) are0.472, 0.478, and 0.477 V.

There are several noteworthy conclusions from these results:

1. Using this more accurate design method (which takes base current intoaccount), circuits designed for a short-circuit current of either 1.0 or10 mA have an optimum operating point at which h_(FE) is stillrelatively large.

2. For both the 1.0 mA and 10 mA circuits, the short-circuit current isonly 2.5% greater than the current at the optimum operating point. Thisvalidates the assumption made in connection with equation 21.

3. For both the 1.0 mA and 10 mA circuits, the collector-emitter drop atthe optimum operating point is about 0.5 V. The value of 1.0 V that wasused in the approximate design method led to the choice of 500Ω forR_(E2). It appears that R_(E2) could have been made somewhat larger orE₂ smaller.

4. The assumptions that the current at the optimum operating point isessentially the same as the short-circuit current and that thecollector-emitter drop at the optimum operating point is small andessentially constant hold true over a wide range of design currentvalues.

For an optimum operating point at 2.0 mA, a circuit should be designedfor a short-circuit current of 2.05 mA, since ##EQU30## Then, ratherthan 4,081Ω as calculated above from equation 8, R₁₂ becomes 4,183Ω.Thus, the value of 4,200Ω that resulted from the approximate method is areasonable design value.

The above numerical results show that Q₂ is close to saturation at theoptimum operating point. For larger values of R_(L), Q₂ will be insaturation, with a collector-emitter voltage, V_(CESAT), of the order of0.1 V. As R_(L) increases, V_(L) increases, becoming nearly equal to E₂when the loop is open.

In saturation, I_(B2) can exceed I_(C2). In fact, I_(C2) will be zerofor a value of I_(B2) that can be determined by solving equation 13,assuming that under these conditions the relationship between V_(BE2)and I_(B2) is the same as that between V_(BE2) and I_(C2) in equation 7.For the design values proposed above, for I_(C2) =0, I_(B2) will be 0.28mA and V_(L) will be 22.7 V.

The same general principles discussed above in connection with on-hooksupply S₂ apply to off-hook supply S₁, with the exception that, whilethe collector voltage of Q₂ is V_(L), that of Q₁ is V_(L+V) _(D), whereV_(D) is the diode voltage. Although the loop current I_(L) is the sumof I_(C1) and I_(C2), when the loop is open and for large values of loopresistance down to the optimum operating point of S₂, D is back-biased,Q₁ is saturated, and I_(C1), is very small. I_(C1) was previouslyassumed to be zero under these conditions for the purpose of selectingthe components of S₂.

When loop resistance is further decreased to the point where D isforward biased, V_(D) is related to I_(C1) by an equation similar toequation 6: ##EQU31##

Here, V_(T) is as defined for equation 6, m is a constant between 1 and2, and V_(K) and I_(K) are device-specific but have values similar tothose given above for Q₂. This forward-bias condition will set in whenV_(L) is slightly less than E₁, at a point V' where I_(C1) =0, as shownin FIG. 8. This point may be calculated by the procedure outlined abovefor the condition I_(C2) =0. As indicated in FIG. 8, when D isforward-biased, its voltage drop of about 0.7 V is subtracted from thecollector voltage that would be calculated according to the methods usedfor designing the on-hook supply. FIG. 8 is to be compared with theideal source characteristic of FIG. 3(a).

Thus, combining FIGS. 6 and 8, the circuit of FIG. 5 has the sourcecharacteristic of FIG. 9, which is to be compared with the idealcharacteristic of FIG. 3(c).

At V', since E₁ is appreciably less than E₂, I_(C2) is essentiallyconstant and equal to its short-circuit value as discussed above. Thus,the optimum operating point can be determined for S₁ independently ofS₂.

Because D is in series with the loop, the condition equivalent toequation 2 is: ##EQU32## However, for the specific design presentedbelow, ##EQU33## while ##EQU34## so very nearly ##EQU35## and theequivalent of equation 18 becomes: ##EQU36## This will be used below todetermine the optimum operating point of off-hook supply S₁.

After I₂ has been chosen to be 2.0 mA, the off-hook supply S₁ isrequired to be optimized for I₁ =18 mA. The optimization process issimilar to that for on-hook supply S₂ as outlined above in conjunctionwith FIG. 6. Here, it seems reasonable to expect an operating range forE₁ of (12±1) V and thus to design for E₁ =11.0 V.

With the loop short-circuited, the diode D will be forward biased sothat both supplies S₁ and S₂ provide current. According to the on-hooksupply example given above, supply S₂ will provide 2.05 mA. Assuming thesame ratio of short-circuit to optimum currents, the off-hook supplyshould provide 18.45 mA. At the operating point of V_(L) =9.0 V, I_(C1)=18.45 mA, diode D will also be conducting. From equation 7, V_(BE1)=0.685 V for I_(C1) =18 mA. Further assuming the same 0.5 V optimumcollector-emitter drop as determined in regard to the on-hook supply,the emitter should be at approximately -10.1 V as shown in FIG. 10. Fora reasonably large value of h_(FE), this is consistent with R_(E1) =50Ω.Assuming a base-emitter drop of 0.7 V, the base will be at -9.4 V, andif R₂₁ is chosen at 2,500Ω, its current will be 3.76 mA, reasonablylarge compared with the base current. Assuming h_(FE) =100, R₁₁ shouldbe [(18 mA) (1.01) (50Ω)+0.7 V]/(3.76 mA-0.18 mA)=450Ω.

The optimization procedure for off-hook supply S₁ follows the same stepsas those utilized for the on-hook supply S₂, with the quantity -A nowtaken as the right hand side of equation 28 instead of equation 18. Thecircuit equations for off-hook supply S₁ are identical to those ofon-hook supply S₂ (equations 3 and 4), with appropriate substitutions.Thus equation 8 may be used to calculate R₁₁ for any desiredshort-circuit current. In order to check whether the conclusions statedin regard to on-hook supply S₂ apply to the design of off-hook supplyS₁, we scale the values of FIG. 10 for a short-circuit current of 10 mA,as follows:

    ______________________________________                                                I.sub.C1MAX  10 mA                                                            R.sub.E1     90Ω                                                        R.sub.21     4,500Ω                                                     V.sub.BE     .670 V                                                           R.sub.11     792Ω                                               ______________________________________                                    

Then, assuming a diode drop of 0.700 V, the same iterative processemployed for the on-hook supply is applied as follows:

    ______________________________________                                        A                      63.6V                                                   ##STR4##              0.00448                                                I.sub.C1MAX /I.sub.1   1.038                                                   ##STR5##               .28V                                                  h.sub.FE (I.sub.1)     79.1                                                   V.sub.1                9.106V                                                 V.sub.1 /I.sub.1      910.6Ω                                            ______________________________________                                    

Just as for the on-hook supply, at the optimum operating point V_(CE) isless than 1 V, h_(FE) is still quite high, and I_(C1MAX) and I₁ are verynearly the same.

Finally, we use the above results to recalculate components of S₁ forh_(FE) =79 and a desired short-circuit current of 1.038 (20.00 mA-2.05mA)=18.63 mA. From equation 8, the value of R₁₁ becomes 463Ω.

The design methods outlined here lead to nominal resistance values shownin FIG. 11. With these values, the circuit marginally satisfies theon-hook and off-hook voltage requirements given in section 2 when E₁ =11V and E₂ =23 V.

On the other hand, the greatest power is drawn when E₁ =13 V, E₂ =25 V,and the loop has zero resistance. For these conditions, current andpower values are calculated using equations 5 and 9:

    ______________________________________                                                     Marginal                                                                              Worst-Case                                                            Loop    Operating                                                             Voltage Power                                                    ______________________________________                                        E.sub.1        11     V      13      V                                        E.sub.2        23     V      25      V                                        I.sub.C1MAX    18.63  mA     24.63   mA                                       I.sub.C2MAX    2.05   mA     2.39    mA                                       I.sub.LMAX     20.68  mA     27.02   mA                                       P.sub.1MAX     246    mW     378     mW                                       P.sub.2MAX     57     mW     71      mW                                       P.sub.1MAX + P.sub.2MAX                                                                      303    mW     449     mW                                       ______________________________________                                    

The circuit presented here meets standard requirements, providing 20 mAfor a 450Ω off-hook loop and 21 V for an on-hook loop. Moreover itpermits the on-hook loop resistance to be much less than the 5Ωstandard, allowing as much as 42 mW to be drawn: 2 mA at 21 V for10,500Ω.

The design example given above assumes that the batteries or voltagesources used have tolerances of±1 V. The design method described hereincan be used to take account of tolerances and environmental variationsin other components as well.

The maximum power dissipated (for a worst-case battery and ashort-circuited loop) is 449 mW. Even a circuit with idealcharacteristics would dissipate 244 mW under these conditions. On theother hand, to meet the 21 V standard even marginally, a conventionalcurrent feed circuit consisting of a battery and current-limitingresistor to provide an optimum resistance of 600Ω would dissipate 735 mW(=21² ÷600). The dual power feed circuit disclosed herein providessubstantial efficiencies in terms of minimizing loop power, which is animportant cost element in providing network services.

It is contemplated to simplify the power feed circuit of the presentinvention, as indicated in FIG. 12, for cases in which it is notrequired to supply an appreciable amount of on-hook power. As seen fromFIG. 12, the off-hook supply S₁ may be unchanged from the circuit shownin FIG. 5, but the on-hook supply is simplified so as to consist only ofthe battery E₂ and a current-limiting resistor R₂. The resistor R₂should be selected to be substantially smaller than the on-hook loadresistance, to provide an operating characteristic shown in idealizedform in FIG. 13.

FIG. 14 is a schematic cross-sectional view of a semi-conductorimplementation in which the transistors Q₁ and Q₂ of FIG. 5 and thediode D are provided in a single integrated circuit, which may alsooptionally include other functions of the SCIU or ONU. In FIG. 14, a psubstrate 60 has formed therein an n region 64 and a p region 62 whichform the diode D. The transistor Q₁ is constituted by the n region 64together with a p region 66 and an n region 68. Terminals 34 and 38 inFIG. 14, which correspond to those shown in FIG. 5, are available toconnect transistor Q₁ to the off-hook bias network of FIG. 5.

The transistor Q₂ is constituted by n region 70, p region 72 and nregion 74. The terminals 46, 50 shown in FIG. 14 correspond to thoseshown in FIG. 5.

Another alternative to the power feed circuit of FIG. 5 (which departsfrom the circuit of FIG. 2, as well) is shown in FIG. 15. In the powerfeed circuit of FIG. 15, the diode D of FIGS. 2 and 5 is replaced by aswitch 80 controlled by a current detector 82. The off-hook supply S₁and on-hook supply S₂ shown in FIG. 15 may be similar to those shown inFIG. 5.

In the circuit of FIG. 15, the switch 80, as controlled by the currentdetector 82, connects the subscriber loop to either off-hook supply S₁or on-hook supply S₂ depending on whether the loop current I_(L) isgreater or less than I₂. In contrast to the circuits of FIGS. 2 and 5,that of FIG. 15 cannot be designed to have a current-voltagecharacteristic in which the loop voltage is a single-valued function ofthe loop current, such as is shown in FIG. 3(c). Rather, in order to bestable at all values of I_(L), including I₂, the characteristic must bedesigned with hysteresis, as is shown in FIG. 16. That is, there must bea finite region of I_(L), from I₂ -ΔI to I₂ +ΔI, where the loop voltagemay assume one of the two values V₁ or V₂.

In the region of hysteresis between I₂ -ΔI and I₂ +ΔI, the value assumedby the loop voltage depends upon its immediately previous history. When,for example, the switch is in the position shown, connecting the loop toon-hook supply S₂ ' the switch must remain in that position until I_(L)reaches I₂ +ΔI before operating to connect the loop to off-hook supplyS₁. When in the latter position, the switch must remain in it untilI_(L) reaches I₂ -ΔI before operating to assume the position shown.

The extent of the region of hysteresis (i.e. the value of ΔI) dependsupon practical considerations regarding tolerances of the currentdetector. If under any conditions the switching thresholds should becomereversed, the switch would oscillate between its two positions forvalues of loop current around I₂.

Thus, it is an advantage of the circuit of FIG. 5 that it is stableagainst oscillation without requiring hysteresis.

Another difference between the circuits of FIGS. 5 and 15 lies in thefact that, in the circuit of FIG. 5 on-hook supply S₂ provides powerduring both on- and off-hook conditions of the subscriber loop, but inthe circuit of FIG. 15, the on-hook supply S₂ only operates when theloop is in an on-hook condition.

In all cases, it is understood that the above-described arrangements aremerely illustrative of the many possible specific embodiments whichrepresent applications of the present invention. Numerous and variedother arrangements can be readily devised in accordance with theprinciples of the present invention without departing from the spiritand scope of the invention.

What is claimed is:
 1. A power feed circuit for a telephone subscriberloop, the loop connected to a unit of terminal equipment, the unit ofterminal equipment being switchable between an on-hook condition and anoff-hook condition; the power feed circuit comprising:a first currentsupply circuit for supplying power to said subscriber loop when saidunit of terminal equipment is in said on-hook condition and when saidunit of terminal equipment is in said off-hook condition; and a secondcurrent supply circuit for supplying power to said subscriber loop onlywhen said unit of terminal equipment is in said off-hook condition.
 2. Apower feed circuit according to claim 1, wherein said first currentsupply circuit includes a first battery and said second current supplycircuit includes a second battery.
 3. A power feed circuit according toclaim 1, wherein said first and second current supply circuits areconnected in parallel to said subscriber loop.
 4. A power feed circuitaccording to claim 3, wherein said second current supply circuit isconnected to said subscriber loop via a diode, and said diode isreverse-biased when said unit of terminal equipment is in said on-hookcondition.
 5. A power feed circuit according to claim 1, furthercomprising a first loop terminal and a second loop terminal, said firstand second loop terminals for connecting said power feed circuit to saidsubscriber loop;wherein said first current supply circuit includes:afirst battery having a positive terminal and a negative terminal, saidpositive terminal connected to said first loop terminal; a firsttransistor having a collector terminal, an emitter terminal and a baseterminal; a diode connected between said second loop terminal of thepower feed circuit and said collector terminal of said first transistor,with said diode having a polarity for conducting current only in thedirection from said second loop terminal to said collector terminal; afirst emitter resistor connected between said emitter terminal of saidfirst transistor and said negative terminal of said first battery; afirst bias resistor connected between said positive terminal of saidfirst battery and said base terminal of said first transistor; and asecond bias resistor connected between said negative terminal of saidfirst battery and said base terminal of said first transistor; andwherein said second current supply circuit includes:a second batteryhaving a positive terminal and a negative terminal, said positiveterminal of said second battery connected to said first loop terminal; asecond transistor having a collector terminal, an emitter terminal and abase terminal, said collector terminal of said second transistorconnected to said second loop terminal of the power feed circuit; asecond emitter resistor connected between said emitter terminal of saidsecond transistor and said negative terminal of said second battery; athird bias resistor connected between said positive terminal of saidsecond battery and said base terminal of said second transistor; and afourth bias resistor connected between said negative terminal of saidsecond battery and said base terminal of said second transistor.
 6. Apower feed circuit according to claim 5, wherein said first battery hasa nominal voltage rating of 12 V and said second battery has a nominalvoltage rating of 24 V.
 7. A power feed circuit for a telephonesubscriber loop, the loop connected to a unit of terminal equipment, theunit of terminal equipment being switchable between an on-hook conditionand an off-hook condition; the power feed circuit comprising:a firstcurrent supply means for supplying power to said subscriber loop whensaid unit of terminal equipment is in said on-hook condition and whensaid unit of terminal equipment is in said off-hook condition; and asecond current supply means for supplying power to said subscriber looponly when said unit of terminal equipment is in said off-hook condition.8. A power feed circuit according to claim 7, wherein said first currentsupply means includes a first battery and said second current supplycircuit includes a second battery.
 9. A power feed circuit according toclaim 7, wherein said first and second current supply means areconnected in parallel to said subscriber loop.
 10. A power feed circuitaccording to claim 9, wherein said second current supply means isconnected to said subscriber loop via a diode, and said diode isreverse-biased when said unit of terminal equipment is in said on-hookcondition.
 11. A power feed circuit according to claim 7, furthercomprising a first loop terminal and a second loop terminal, said firstand second loop terminals for connecting said power feed circuit to saidsubscriber loop;wherein said first current supply means includes:a firstbattery having a position terminal and a negative terminal, saidpositive terminal connected to said first loop terminal; a firsttransistor having a collector terminal, an emitter terminal and a baseterminal; a diode connected between said second loop terminal of thepower feed circuit and said collector terminal of said first transistor,with said diode having a polarity for conducting current only in thedirection from said second loop terminal to said collector terminal; afirst emitter resistor connected between said emitter terminal of saidfirst transistor and said negative terminal of said first battery; afirst bias resistor connected between said positive terminal of saidfirst battery and said base terminal of said first transistor; and asecond bias resistor connected between said negative terminal of saidfirst battery and said base terminal of said first transistor; andwherein said second current supply means includes:a second batteryhaving a positive terminal and a negative terminal, said positiveterminal of said second battery connected to said first loop terminal; asecond transistor having a collector terminal, an emitter terminal and abase terminal, said collector terminal of said second transistorconnected to said second loop terminal of the power feed circuit; asecond emitter resistor connected between said emitter terminal of saidsecond transistor and said negative terminal of said second battery; athird bias resistor connected between said positive terminal of saidsecond battery and said base terminal of said second transistor; and afourth bias resistor connected between said negative terminal of saidsecond battery and said base terminal of said second transistor.
 12. Apower feed circuit according to claim 11, wherein said first battery hasa nominal voltage rating of 12 V and said second battery has a nominalvoltage rating of 24 V.
 13. A method of feeding power to a telephonesubscriber loop, the loop connected to a unit of terminal equipment, theterminal equipment being switchable between an on-hook condition and anoff-hook condition; the method comprising:supplying power to saidsubscriber loop with a first current supply circuit when said unit ofterminal equipment is in an on-hook condition and when said unit ofterminal equipment is in said off-hook condition; and supplying power tosaid subscriber loop with a second current supply circuit only when saidunit of terminal equipment is in said off-hook condition.